Syntheses And Properties Of CdSe And Other Semiconductor Nanocrystals

With the development of science and technology, more and more researchers are paying attention to the nano-sized materials. These materials possess lots of special properties, such as, Kubo theory, quantum confinement effect, small size effect, surface effect, etc. Recently, colloidal semiconductor nanocrystals, one of the most important members of nanomaterial families, have attracted much attention in both fundamental studies and industrial applications. Studying these nanocrystals, the researchers have discovered many significant phenomena. However, in this field there still are lots of problems to be dealt with, such as few reports on the growth kinetics of semiconductor nanocrystals, scarce attempts to explore the green approaches and products, how to simply obtain magic-sized nanocrystals, and serious lack of detailed studies on the nanocrystal temperature-dependent properties, and so on. In this thesis, we focus on CdSe, ZnSe, PbSe, SnSe, and some other semiconductor nanocrystals, discuss their growth kinetics, explore their green synthetic routes, characterize their shapes, structures, and optical properties, and study their temperature-dependent band gap.We have synthesized size-controlled and nearly monodisperse CdSe nanocrystals in the noncoordinating solvent octadecene protected under the inert gas, and detailedly studied the effect of both ligand and temperature on their growth kinetics, particle size, and size distribution. This study can optimize the conditions of synthesizing high-quality CdSe nanocrystals, find the optimal balance between nucleation and growth, and delay Ostwald ripening. The experimental results show that excessive ligands and Se favor the synthesis of relatively larger and more monodisperse CdSe nanocrystals.In the synthesis of CdSe nanocrystals performed in a system with oleic acid and octadecene as the primary ligand and the noncoordinating solvent, respectively, the addition of a secondary ligand trioctylphosphine oxide (TOPO) can not only improve the size distribution of CdSe nanocrystals, but also tune the balance between nucleation and growth. This tunable balance allow us to facilely and reproducibly synthesize monodisperse (~6%) and small (<2.3 nm in diameter) CdSe nanocrystals, which are difficult or even impossible using the previously reported approaches. After studying the nanocrystal growth, we find that the well-established nanocrystal growth mechanism, in which nucleation is followed by focusing of size distribution and ended with defocusing of size distribution, can not be applied to our reaction system. Instead, we observe an exceptional type of growth mechanism: after nucleation, clear defocusing instead of focusing follows; then slight focusing occurs. This exceptional growth mechanism is attributed to the TOPO-induced transformation in small CdSe nanocrystals.We have further demonstrated the reaction system in octadecene can be also performed under ambient conditions, where size-controlled CdSe nanocrystals with visible emissions have been synthesized. The as-synthesized CdSe nanocrystals, characterized by absorption spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, and X-ray diffraction, have narrow size distributions without any postsynthetic size-selective procedure. Further experimental results show that, by simply changing the amount of trioctylphosphine (the ligand of selenium source), the monomer reactivity can be easily tuned. Such tunable reactivity is much more dramatic than the one by altering the concentration of oleic acid (the ligand of cadmium precursor).We have introduced a facile and reproducible approach for the synthesis of magic-sized CdSe and CdTe nanocrystals via the injection of the Se or Te precursor into the phenyl ether media containing the cheap cadmium-oleate precursor. The resulting CdSe nanocrystals, which are studied representatively, exhibit strong and fixed absorption features with unusually narrow emission properties. White-light emission can be achieved by two different routes. One is to mix colors emitted from both the magic-sized nanocrystals and the subsequently transformed regular-sized ones; the other is to expose the magic-sized nanocrystals to ambient conditions. During the exposure, the uncommon continuity in CdSe nanocrystal sizes below 2 nm can be also observed. A systematic study of the nanocrystal formation process shows that monomer activity and injection/growth temperatures are important parameters to the growth kinetics of magic-sized nanocrystals. Variation of these parameters provides tunable existence periods of nanocrystals in the hot solution. Moreover, study of these nanocrystals would make them potential materials for white-light applications, and provide the impetus for developing more economical and environmentally friendly materials with similar properties.We have reported experimental results on the temperature dependence of band gap in size-controlled CdSe nanocrystals. Such temperature-dependent property is also size-dependent due to quantum confinement effect. Within a certain temperature range, the band gap absorption peak position (λin nm) of CdSe nanocrystals with a given particle size (D in nm) can be expressed as a simple linear function of temperature (T in K):λ= 0.1 T + C (D, 298 K). A comparison reveals that the band gap values calculated using this function are in agreement with the experimental ones reported previously. Therefore, this work should be helpful in estimating the potential color changes of the output light from CdSe nanocrystals, when the nanocrystals work in different temperature environments. Also, this study should be helpful in exploring the temperature-induced band gap variations of other semiconductor nanocrystals.A nontoxic, simple, cheap and reproducible strategy, which completely meets the standard of green chemistry, is introduced for the synthesis of green ZnSe nanocrystals. Importantly, the production of these high-quality nanocrystals can be readily scaled up and performed directly under ambient conditions without deteriorating the nanocrystal quality. The experimental results show that the zinc blende ZnSe nanocrystals have narrow size distributions without any postsynthetic size-selective procedure. A systematic study of the nanocrystal formation process indicates that the amount of precursors is a more sensitive parameter on determining the nanocrystal size than other reaction parameters. Variation of this parameter provides the tunable sizes of nanocrystals in the high-temperature annealing process. This tunability is interpreted well by the growth kinetics.We have explored the synthesis of comparatively less-known IV-VI semiconductor nanocrystals that have received much attention in recent years. The synthetic strategy for CdSe nanocrystals described in Chapter 2 is found available for the synthesis of size-controlled PbSe nanocrystals. Experimental results show that the shapes of PbSe nanocrystals are size-dependent, that is, dotlike PbSe nanocrystals gradually become cubic with the increase of their size. Additionally, we attempt to synthesize the little reported SnSe and SnS nanocrystals and obtain some initial experimental results.